Prior art autofocus systems, even those which form part of a system performing automatic pattern recognition on the objects focused, are not able to identify specific objects or types of objects while performing the focusing operation. This deficiency may permit them to focus on dust particles, scratches, or other artifacts, or to miss recognizing an object of interest because it was not in focus when automatic pattern recognition was performed. Prior art systems may focus on irrelevant details because they possess high frequency content and occur in the same region as an object of interest. The result is that a prior art system may miss an opportunity to focus on and identify an important feature, such as the nucleus of a cell.
Such problems are particularly acute when the field of view addressed by a single focusing operation is large, containing many objects of interest. There may be too many objects for time to allow independently focusing a system on each one. Yet a single focusing operation, encompassing the entire field, may not focus on the actual objects of interest, but instead on some irrelevant features, simply because they are large, or have a large high frequency content.
The need for accurate focusing of an automated optical system becomes critical when the system is designed to detect rare events, such as, for example, pre-cancerous cells occurring in a Pap smear. In the case of a Pap smear, a relatively enormous area must be scanned very quickly at low magnification. In one embodiment of an automated Pap smear scanning system, for instance, a 4.times. microscope objective, covering a field of view which is 1.4 mm square, is used for a low magnification scan. The area so scanned comprises the entire region under the coverslip of the smear, which may contain over 700 such fields of view.
The purpose of low magnification scanning is to identify possible pre-cancerous cells, comprising a tiny percentage of the smear, which need to be re-examined at high magnification. At low magnification, hundreds of objects may appear in a single field of view, so independently focusing on each one of them is not feasible. But if the system focuses on irrelevant matter rather than identifiable cell nuclei, the one pre-cancerous cell in a thousand may not be selected for examination at high magnification, resulting in a failure to detect a pre-cancerous condition.
Thus, it is critical for such a system to focus reliably and specifically on the objects of interest, and not on irrelevant features or artifacts in the specimen. Artifacts present in a Pap smear specimen may include flecks of graphite from the pencil used to mark the microscope slide, tiny splinters of wood from the instrument used to collect the specimen, blood, hair, strands of mucus, as well as thick clumps of cellular matter which may be unsuitable for examination by conventional transmission microscopy. A successful Pap smear scanning instrument must reliably focus on cell nuclei of interest, and not on this less relevant background of artifacts.